Your computer will not boot returning the error "Unmountable boot Volume".
Access the recovery console. (This is the first repair option you get after you boot off the Windows XP CD (To see where this menu is Click here), unfortunately some OEM CD's (Or recovery CD's) do not have this option)
Once you log into the recovery console type the following command "chkdsk /f". Once the scan is complete then reboot the computer. You should be able to log into Windows.
This option tends to be very successful.
Quite simply take the Hard Drive out of your computer, then go to a friends house, that is a friend who owns a Windows XP computer, and with permission open up his/hers computer, disconnect their CD-Rom drives, and plug your hard drive in with theirs, of course make sure the cable is seated correctly and that the power connector is plugged in.
Your friend computer will now boot up and run Chkdsk before it logs into Windows, let this run & complete.
Take your Hard Drive home and test. This method has a good success rate.